Playa Cerro Gordo: Come for the (clean, uncrowded) beach, stay for the hike that brings you to a breathtaking view, unobstructed by anything man-made.
Cueva de las Golondrinas & Poza de los Mujeres: Take a nice long walk to get away from civilization and reward yourself with a day of exploring a few caves at low tide and/or laying out on the beach.
El Yunque: The only rain forest in the US National Forest System and home to stunning vistas and nearly private waterfalls. (Bring swimsuits and water shoes!)
Cueva del Indio: Climb down an old (but sturdy) wooden ladder to see Taino carvings in the rocks. Stay up top to enjoy (yet another) breathtaking view.
Rio Camuy Cave Park: This guided tour takes you into a natural limestone cavern formed by the Camuy River. Watch out for bat poop!